Onion

Choosing

Dry, papery skin and a firm body. A thick or soft neck means it has started to go. Softer onions hold more water and take longer to cook down.

Cooking time makes it a different ingredient

• 3–5 minutes — still crunchy, still sharp. The baseline for stir-fries.
• 10–15 minutes — translucent and sweet. The foundation of curries and stews.
• 30–40 minutes — caramelised and brown. The forty minutes in French Onion Soup is this. There is no shortcut: raising the heat burns them rather than speeding them up.

The difference between caramelisation and the Maillard reaction is set out in Browning.

Using it raw

To take the edge off, slice it and soak it in cold water for 5–10 minutes, then drain — the sulphur compounds wash out. Macerating in salt and acid works too; the onion salsa served with Pique Macho does exactly that.

Red onion holds its colour and sweetness raw, which suits Greek Salad and sandwiches.

Why it stings, and what helps

Volatile sulphur compounds released by cutting irritate the tear ducts. The two things that actually help are chilling it (30 minutes in the fridge) and using a sharp knife. A dull blade crushes more cells and makes it far worse — see Knife skills.

Storing

Dark, ventilated, room temperature. Stored with potatoes they spoil each other faster. Cut onion keeps sealed in the fridge for three or four days. Freezing destroys the crunch, so freeze only what you will cook.

Substitutes

Shallots are milder and more delicate (one onion ≈ 3–4 shallots). The white part of spring onion works, but it will not caramelise.